Coatings for metal cans used for packaging of refreshing beverages, processed foods, etc. must have water resistance at high temperatures. As such coatings, there have heretofore been used organic solvent type coatings capable of forming a coating film exhibiting excellent properties when heat-cured, and examples thereof include epoxy/amino resins, acrylic/amino resins and polyester/amino resins.
With the organic solvent type coatings, however, a large amount of an organic solvent vaporizes into the air when they are applied, which has invited environmental pollution and a waste of resources.
Hence, aqueous coatings using water as a medium have been investigated recently. Conventional aqueous coatings, however, have low water resistance. That is, their coating films show significant reduction in hardness and impact strength when subjected to a treatment with pressurized steam of 120.degree. C. or higher (a retort treatment) and are unusable in practical applications, although said coating films are resistant to a treatment with hot water (e.g. boiling water).
In order to develop an aqueous coating resistant to the retort treatment, Japanese Patent Application Kokai (Laid-Open) No. 72577/1991 and U.S. Pat. No. 5,096,954 propose an aqueous coating composition comprising (a) an aqueous acrylic resin obtained by neutralizing, with a base, a copolymer which is composed mainly of an aromatic vinyl monomer and an alkyl (meth)acrylate and which contains carboxyl groups and hydroxyl groups in the molecule, (b) an amino resin and (c) a polyol which is a polyester, a polyether, a polyurethane or a polybutadiene each having hydroxyl groups at the ends.
Further, EP 0526991A1 proposes an aqueous coating composition comprising the above resin (a), the above resin (b) and (d) a reaction product of an amino compound and an epoxy resin.
Each of the above aqueous coating compositions employs a means for increasing the crosslinking density of the aqueous acrylic resin in order to obtain a cured coating film of high water resistance and uses the resin (c) or (d) in order to obtain a tough coating film.